Many modern telephones have a "hands-free" or "speaker-phone" mode which allows the user to talk and listen without using a handset. Often the person on the other end of the call perceives the hands-free user as "far away" or "in a barrel." This is often called the barrel effect and is caused by reflections and reverberations of the voice reaching the microphone at different times. The strength of the barrel effect is proportional to the distance of the talker from the speaker-phone. Another common problem which occurs when the microphone is located far from the talker is that of loudspeaker-to-microphone coupling. This can result in instabilities such as audio feedback or "howling." In the case of duplex systems, loudspeaker-to-microphone coupling can limit the performance of the automatic transmit/receive switch. These problems are also encountered in teleconference systems, talker podium microphones, and stage performance microphone systems. It is inconvenient to pass microphones around to individual talkers, or to issue microphones to every participant.
It is known that the use of a directional microphone aimed in the direction of the talker will reduce the barrel effect. An example of such a directional microphone is a microphone array as disclosed in Wallace, Jr., U.S. Pat. No. 4,311,874 "Teleconference Microphone Arrays", which is herein incorporated by reference. See also H. Nomura, H. Miyata, and T. Houtgast, "Microphone Arrays for Improving Speech Intelligibility in a Reverberant or Noisy Space", J. Audio Eng. Soc., Vol. 41, No. 10, pp. 771-781. A microphone array is a multichannel acoustic acquisition setup consisting of a collection of sensors placed in different points in space, in order to spatially sample the propagating sound pressure field. With the use of such a microphone array, sounds coming from the talker are picked up more strongly than the reflections and reverberations which generally arrive from different directions. However, a good balance needs to be found between directionality and acceptance angle. If the talker strays into an area where the directional microphone is not aimed, the signal level will drop and the microphone will emphasize the barrel effect rather than reduce it. The microphone directional patterns must therefore be wide enough to accommodate the angles generally occupied by the talker. On the other hand, the wider the acceptance angle, the more reflections and reverberations are picked up and the directional microphone becomes less effective in reducing the barrel effect.
In most conferencing audio systems, several microphones (often directional) are used to cover zones in the room where the conference is held. This is often the case when more than one talker is involved. In most systems, the signal is then taken only from the microphone which has the strongest signal. The other microphones, which would be picking up reflections and reverberations of the talker, are momentarily switched off. In some cases, a human operator is responsible for switching between microphones.
Microphone arrays have the capability to detect and track the position of an acoustic source and can be automatically steered to provide a directional acquisition of the desired signal. Signal processing techniques allow a microphone array to be steered toward a particular source without changing the position of the sensors. These techniques consist of delaying, filtering and adding the outputs in such a way to achieve the desired spatial selectivity.
A standard technique for acoustic aiming is to steer the microphone in the direction of the loudest sound. An example of the use of this type of directional microphone is Addeo, U.S. Pat. No. 5,335,011 in which there is described a microphone array that selectively forms a highly directional beam in response to a sound originating from a particular location. Under the control of a microprocessor, the microphone array scans volume zones in the room where it is located. If a sound is detected in a particular zone, the microphone array control device causes the microphone array to maintain a beam in that particular zone. This provides or reduced ambient noise, room reverberation and acoustic coupling.
Another example of a steerable microphone array is that disclosed in J. L. Flanagan, J. D. Johnston, R. Zahn, and G. W. Elko, "Computer-steered microphone arrays for sound transduction in large rooms" J. Acoust. Soc. Am. 78 (5), November 1995, pp. 1508-1518. In this system, speech detection algorithms under microprocessor control are used to detect, locate, and point the microphone array to the desired source.
Additionally, there are prior art systems that employ microphone arrays coupled to video cameras, where the microphone array is used to determine the point of origin of a sound, and to direct a camera at that sound source. These systems (also known as "smart cameras") are used to eliminate the need for a human camera operator in an auditorium or conference hall environment.
The main disadvantage of the prior art microphone array systems is that they only work well in auditoriums that have been specially designed to enhance acoustics. In ordinary offices, conference rooms or teleconference rooms, the reflections and reverberations impede the accuracy of the microphone array to accurately locate the point of origin of a sound source. In addition, the direction of the loudest sound can be different for different frequency ranges. As well, even where the aim of the microphone array is correct, the decision to point the microphone array in a different direction in response to a new talker is not instantaneous and often the first part of that talker's words are cut off, or "clipped".